Subject: Re: [PATCH v21 020/100] c/r: documentation

From: Oren Laadan <[email-masked]>
Date: Fri, 07 May 2010 02:54:42 -0400

Thanks for reading carefully through and pointing out
glitches and inconsistencies. I'll fix it for next post.

Oren.

On 05/06/2010 04:27 PM, Randy Dunlap wrote:
> On Sat,  1 May 2010 10:15:02 -0400 Oren Laadan wrote:
> 
>> Covers application checkpoint/restart, overall design, interfaces,
>> usage, shared objects, and and checkpoint image format.
>>
>> Signed-off-by: Oren Laadan <[email-masked]>
>> Signed-off-by: Dave Hansen <[email-masked]>
>> Acked-by: Serge E. Hallyn <[email-masked]>
>> Tested-by: Serge E. Hallyn <[email-masked]>
>> ---
>>  Documentation/checkpoint/checkpoint.c      |   38 +++
>>  Documentation/checkpoint/readme.txt        |  370 ++++++++++++++++++++++++++++
>>  Documentation/checkpoint/self_checkpoint.c |   69 +++++
>>  Documentation/checkpoint/self_restart.c    |   40 +++
>>  Documentation/checkpoint/usage.txt         |  247 +++++++++++++++++++
>>  5 files changed, 764 insertions(+), 0 deletions(-)
>>  create mode 100644 Documentation/checkpoint/checkpoint.c
>>  create mode 100644 Documentation/checkpoint/readme.txt
>>  create mode 100644 Documentation/checkpoint/self_checkpoint.c
>>  create mode 100644 Documentation/checkpoint/self_restart.c
>>  create mode 100644 Documentation/checkpoint/usage.txt
> 
>> diff --git a/Documentation/checkpoint/readme.txt b/Documentation/checkpoint/readme.txt
>> new file mode 100644
>> index 0000000..4fa5560
>> --- /dev/null
>> +++ b/Documentation/checkpoint/readme.txt
>> @@ -0,0 +1,370 @@
>> +
> ...
>> +In contrast, when checkpointing a subtree of a container it is up to
>> +the user to ensure that dependencies either don't exist or can be
>> +safely ignored. This is useful, for instance, for HPC scenarios or
>> +even a user that would like to periodically checkpoint a long-running
> 
>                who
> 
>> +batch job.
>> +
> ...
> 
>> +
>> +Checkpoint image format
>> +=======================
>> +
> ...
> 
>> +
>> +The container configuration section containers information that is
> 
>                                        contains
> 
>> +global to the container. Security (LSM) configuration is one example.
>> +Network configuration and container-wide mounts may also go here, so
>> +that the userspace restart coordinator can re-create a suitable
>> +environment.
>> +
> ...
> 
>> +
>> +Then the state of all tasks is saved, in the order that they appear in
>> +the tasks array above. For each state, we save data like task_struct,
>> +namespaces, open files, memory layout, memory contents, cpu state,
> 
>                                                            CPU (throughout, please)
> 
>> +signals and signal handlers, etc. For resources that are shared among
>> +multiple processes, we first checkpoint said resource (and only once),
>> +and in the task data we give a reference to it. More about shared
>> +resources below.
>> +
> ...
> 
>> +
>> +Shared objects
>> +==============
>> +
>> +Many resources may be shared by multiple tasks (e.g. file descriptors,
>> +memory address space, etc), or even have multiple references from
> 
>                          etc.),
> 
>> +other resources (e.g. a single inode that represents two ends of a
>> +pipe).
>> +
> ...
> 
>> +Memory contents format
>> +======================
>> +
>> +The memory contents of a given memory address space (->mm) is dumped
> 
>                                                               are (I think)
> 
>> +as a sequence of vma objects, represented by 'struct ckpt_hdr_vma'.
>> +This header details the vma properties, and a reference to a file
>> +(if file backed) or an inode (or shared memory) object.
>> +
>> +The vma header is followed by the actual contents - but only those
>> +pages that need to be saved, i.e. dirty pages. They are written in
>> +chunks of data, where each chunks contains a header that indicates
> 
>                               chunk
> 
>> +that number of pages in the chunk, followed by an array of virtual
> 
>    the
> 
>> +addresses and then an array of actual page contents. The last chunk
>> +holds zero pages.
>> +
> ...
> 
>> +Kernel interfaces
>> +=================
>> +
>> +* To checkpoint a vma, the 'struct vm_operations_struct' needs to
>> +  provide a method ->checkpoint:
>> +    int checkpoint(struct ckpt_ctx *, struct vma_struct *)
>> +  Restart requires a matching (exported) restore:
>> +    int restore(struct ckpt_ctx *, struct mm_struct *, struct ckpt_hdr_vma *)
>> +
>> +* To checkpoint a file, the 'struct file_operations' needs to provide
>> +  the methods ->checkpoint and ->collect:
>> +    int checkpoint(struct ckpt_ctx *, struct file *)
>> +    int collect(struct ckpt_ctx *, struct file *)
>> +  Restart requires a matching (exported) restore:
>> +    int restore(struct ckpt_ctx *, struct ckpt_hdr_file *)
>> +  For most file systems, generic_file_{checkpoint,restore}() can be
>> +  used.
>> +
>> +* To checkpoint a socket, the 'struct proto_ops' needs to provide
> 
>      To checkpoint/restart a socket,
> 
>> +  the methods ->checkpoint, ->collect and ->restore:
>> +    int checkpoint(struct ckpt_ctx *ctx, struct socket *sock);
>> +    int collect(struct ckpt_ctx *ctx, struct socket *sock);
>> +    int restore(struct ckpt_ctx *, struct socket *sock, struct ckpt_hdr_socket *h)
> 
> 
>> diff --git a/Documentation/checkpoint/usage.txt b/Documentation/checkpoint/usage.txt
>> new file mode 100644
>> index 0000000..c6fc045
>> --- /dev/null
>> +++ b/Documentation/checkpoint/usage.txt
>> @@ -0,0 +1,247 @@
>> +
>> +	      How to use Checkpoint-Restart
>> +	=========================================
>> +
>> +
>> +API
>> +===
>> +
>> +The API consists of three new system calls:
>> +
>> +* long checkpoint(pid_t pid, int fd, unsigned long flag, int logfd);
> 
>                                                       flags,
> 
>> +
>> + Checkpoint a (sub-)container whose root task is identified by @pid,
>> + to the open file indicated by @fd. If @logfd isn't -1, it indicates
>> + an open file to which error and debug messages are written. @flags
>> + may be one or more of:
>> +   - CHECKPOINT_SUBTREE : allow checkpoint of sub-container
>> + (other value are not allowed).
>> +
>> + Returns: a positive checkpoint identifier (ckptid) upon success, 0 if
>> + it returns from a restart, and -1 if an error occurs. The ckptid will
>> + uniquely identify a checkpoint image, for as long as the checkpoint
>> + is kept in the kernel (e.g. if one wishes to keep a checkpoint, or a
>> + partial checkpoint, residing in kernel memory).
>> +
>> +* long sys_restart(pid_t pid, int fd, unsigned long flags, int logfd);
>> +
>> + Restart a process hierarchy from a checkpoint image that is read from
>> + the blob stored in the file indicated by @fd.  If @logfd isn't -1, it
>> + indicates an open file to which error and debug messages are written.
>> + @flags will have future meaning (must be 0 for now). @pid indicates
>> + the root of the hierarchy as seen in the coordinator's pid-namespace,
>> + and is expected to be a child of the coordinator. @flags may be one
>> + or more of:
>> +   - RESTART_TASKSELF : (self) restart of a single process
>> +   - RESTART_FROEZN : processes remain frozen once restart completes
> 
>                 FROZEN ?
> 
>> +   - RESTART_GHOST : process is a ghost (placeholder for a pid)
> 
> about @flags:  Above says both of these:
> a) @flags will have future meaning (must be 0 for now)
> b) @flags may be one or more of:
> 
> so please decide which one it is ;)
> 
>> + (Note that this argument may mean 'ckptid' to identify an in-kernel
>> + checkpoint image, with some @flags in the future).
>> +
>> + Returns: -1 if an error occurs, 0 on success when restarting from a
>> + "self" checkpoint, and return value of system call at the time of the
>> + checkpoint when restarting from an "external" checkpoint.
>> +
> ...
>> +
>> +Sysctl/proc
>> +===========
>> +
>> +/proc/sys/kernel/ckpt_unpriv_allowed		[default = 1]
>> +  controls whether c/r operation is allowed for unprivileged users
> 
>                       C/R
> 
>> +
>> +
>> +Operation
>> +=========
>> +
>> +The granularity of a checkpoint usually is a process hierarchy. The
>> +'pid' argument is interpreted in the caller's pid namespace. So to
>> +checkpoint a container whose init task (pid 1 in that pidns) appears
>> +as pid 3497 the caller's pidns, the caller must use pid 3497. Passing
>> +pid 1 will attempt to checkpoint the caller's container, and if the
>> +caller isn't privileged and init is owned by root, it will fail.
>> +
>> +Unless the CHECKPOINT_SUBTREE flag is set, if the caller passes a pid
>> +which does not refer to a container's init task, then sys_checkpoint()
>> +would return -EINVAL.
> 
>    returns -EINVAL.
> 
> ...
> 
>> +
>> +
>> +User tools
>> +==========
>> +
>> +* checkpoint(1): a tool to perform a checkpoint of a container/subtree
>> +* restart(1): a tool to restart a container/subtree
>> +* ckptinfo: a tool to examine a checkpoint image
>> +
>> +It is best to use the dedicated user tools for checkpoint and restart.
>> +
>> +If you insist, then here is a code snippet that illustrates how a
>> +checkpoint is initiated by a process inside a container - the logic is
>> +similar to fork():
>> +	...
>> +	ckptid = checkpoint(0, ...);
>> +	switch (crid) {
> 
> 	       (ckptid) ?
> 
>> +	case -1:
>> +		perror("checkpoint failed");
>> +		break;
>> +	default:
>> +		fprintf(stderr, "checkpoint succeeded, CRID=%d\n", ret);
> 
> s/ret/ckptid/ ?
> 
>> +		/* proceed with execution after checkpoint */
>> +		...
>> +		break;
>> +	case 0:
>> +		fprintf(stderr, "returned after restart\n");
>> +		/* proceed with action required following a restart */
>> +		...
>> +		break;
>> +	}
>> +	...
>> +
>> +And to initiate a restart, the process in an empty container can use
>> +logic similar to execve():
>> +	...
>> +	if (restart(pid, ...) < 0)
>> +		perror("restart failed");
>> +	/* only get here if restart failed */
>> +	...
>> +
>> +Note, that the code also supports "self" checkpoint, where a process
> 
>    Note that
> 
>> +can checkpoint itself. This mode does not capture the relationships of
>> +the task with other tasks, or any shared resources. It is useful for
>> +application that wish to be able to save and restore their state.
> 
>    applications
> 
>> +They will either not use (or care about) shared resources, or they
>> +will be aware of the operations and adapt suitably after a restart.
>> +The code above can also be used for "self" checkpoint.
>> +
>> +
>> +You may find the following sample programs useful:
>> +
>> +* checkpoint.c: accepts a 'pid' and checkpoint that task to stdout
> 
>                                        checkpoints
> 
>> +* self_checkpoint.c: a simple test program doing self-checkpoint
>> +* self_restart.c: restarts a (self-) checkpoint image from stdin
>> +
>> +See also the utilities 'checkpoint' and 'restart' (from user-cr).
>> +
>> +
>> +"External" checkpoint
>> +=====================
>> +
>> +To do "external" checkpoint, you need to first freeze that other task
>> +either using the freezer cgroup.
> 
> eh?  cannot parse that.
> 
>> +
>> +Restart does not preserve the original PID yet, (because we haven't
>> +solved yet the fork-with-specific-pid issue). In a real scenario, you
>> +probably want to first create a new names space, and have the init
> 
>                                        namespace,
> 
>> +task there call 'sys_restart()'.
>> +
>> +I tested it this way:
> 
> ...
> 
> ---
> ~Randy
> *** Remember to use Documentation/SubmitChecklist when testing your code ***
> 

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